To evaluate the optical characteristics of an optical component such as a lens or a prism, the aberration of the optical component is inspected by the use of an interference of light beams. FIG. 23 shows a Mach-Zehnder radial shearing interferometer 1000 which is one of the known techniques for inspecting the characteristics of optical components by the use of the interference of light beams. Referring to the drawing, light 1002 emitted from a light source (not shown) passes through a lens 1004 to be inspected, and a lens 1006, and then is divided by a half mirror 1008, into two light beams 1010 and 1012. The light beam 1010 having passed through the half mirror 1008 reflects on a mirror 1014 and a half mirror 1016 and enters a lens 1018. On the other hand, the light beam 1012 having reflected on the half mirror 1008 further reflects on a half mirror 1020, passes through a converging lens 1022, a pin hole 1024, a lens 1026 and the half mirror 1016, and enters the lens 1018. The two light beams 1010 and 1012 are superposed on the lens 1018 to form an interference figure. The interference figure is projected to an image pickup device 1028. Information contained in the interference figure is transmitted from the image pickup device 1028 to an analyzer in the form of electronic signal where it is analyzed to evaluate the wave front aberration, namely, the optical characteristics of the optical component or the lens 1004.
The mirror 1014 of the interferometer 1000 is connected to a drive mechanism 1034 for moving the mirror 1014 in the direction indicated by arrow 1032. Typically, the drive mechanism 1034 intermittently moves the mirror 1014 at small intervals of one several tenth of the wavelength of light. A brightness variation of the interference figure received by the image pickup device 1028 is schematically illustrated in FIGS. 24A-24D, obtained by moving the mirror 1014 using the drive mechanism 1034, in the optical axis direction, i.e., light traveling direction at intervals of one forth of the light wavelength λ/4 (λ: light wavelength). As can be seen from the drawings, the brightest interference figure is obtained when the difference in optical path length between the two light beams is nλ [n: an integer] (see FIG. 24D). The darkest interference figure is obtained when the difference in optical path length between the two light beams is (1/2+n)λ (see FIG. 24B). Medium bright interference figures are obtained when the difference in optical path length between the two light beams is (1/4+n)λ or (3/4+n)λ (see FIGS. 24A and 24C). The relationship between the difference of the optical path length and the brightness of the interference figure is shown in the following Table 1.
TABLE 1Relationship between Difference in Optical Path Lengthand Brightness of Interference FigureDifference in(¼ + n) λ(½ + n) λ(¾ + n) λ(n) λoptical path lengthBrightnessMediumDarkMediumBrightof interferencefigureCorresponding FigureFIG. 24AFIG. 24BFIG. 24CFIG. 24Dn: an integerλ: the wavelength of light
Using the characteristics, the analyzer 1030 obtains necessary information from the interference figures each formed by two light beams having a predetermined phase difference therebetween, and substitutes the obtained information into Zernike Polynomial to thereby evaluate the aberration from the coefficients of the Polynomial.
Another light interference evaluation technique, a phase shift method, is disclosed in Documents 1 and 2. Referring to FIGS. 25, 26A, 26B, 27A, 27B and 28, according to the phase shift method, one wave front (1100) out of two wave fronts to be interfered with each other is moved relative to the other wave front (1102), which causes the interfering conditions to change with the advance of the wave front 1100 and thereby to vary the brightness of the interference figure and the intensity of light change sinusoidally. If the optical system has no aberration, the light intensities at two spaced points X and Y in the interference figure change sinusoidally, respectively, in which no phase difference exists between the changes of both light intensities. If on the other hand the optical system has a certain aberration, a phase difference Δ exists between the two sinusoidal changes in light intensity.
Non-patent Document 1:
OYOUKOUGAKU HIKARI KEISOKU NYUMON by Toyohiko Tanitakai, Maruzen, 1988, p 131
Non-patent Document 2:
Principles of Optical components by M. Born and E. Wolf, Tokaidaigaku Shuppan, 1995, p 69
Typically, the light intensity of an interference figure or interference fringes may change due to not only an interference but also another factor such as variation in the sensitivity of a camera or distribution of light intensities of an original wave front. The change of light intensity due to another factor are evaluated by the phase shift method, so that only the change of light intensity due to an interference can be expressed in the form of a phase change or phase pattern. Specifically, the phase pattern is determined by receiving the interference figure obtained when the difference in wave fronts of two beams equals to one wavelength, by the use of Discrete Fourier Transform. A phase pattern may be obtained without using the Fourier transform, by appropriately using the phase advancement between two wave fronts and the number of figures picked up.
The interferometer using the phase shift method has two problems. One is the need of precisely moving an optical component such as a mirror with a slow motion mechanism using, for example, a piezoelectric element. The other is the need of causing a larger difference in optical path length between two wave fronts to be interfered with each other.
A Patent Document 1 discloses an interference fringe-analyzing technique which solves the former problem. According to this technique, parallel light is transmitted into a half mirror which is obliquely disposed to the traveling direction of the parallel light, where it is divided into a first light beam which passes through the half mirror and a second light beam which reflects on the half mirror. The first light beam is reflected on a member to be evaluated and again transmitted into the half mirror where it is reflected and then is received by an image pickup device. The second light beam having reflected on the half mirror is further reflected on a reference surface and transmitted through the half mirror and is finally received by the image pickup device. The image pickup device picks up an interference figure formed by the first and second light beams. Information of the interference figure picked up are transmitted in the form of electronic signal to an image input board where the interference figure is analyzed by the use of Fourier Transform to evaluate the aberration of the subject member.
Patent Document 1: JP 2001-227907 A
Patent Document 2 discloses the shearing interference optical system which solves the latter problem. According to the shearing interference optical system, light having passed through a lens to be evaluated enters a diffraction grating. The diffraction grating emits diffracted beams so an interference figure of the diffracted beams of different orders such as 0th-order and +1st-order diffracted beams or 0th order and −1st-order diffracted beams is projected onto the image pickup device. During the image pickup operation, the diffraction grating is moved in a direction orthogonal to the grooves of the grating. As a result, the light intensity of the interference figure changes due to the change of the distance between the wave fronts of the two diffracted rays interfered with each other. Then, the change of light intensity is evaluated by the phase shift method so as to evaluate the aberration of the lens.
Patent Document 2: JP 2000-329648 A
Each of the Mach-Zehnder radial shearing interferometer and the interferometer using the interference fringe-analyzing method, however, requires a sufficiently large light path difference between two light beams and thereby can suffer from an adverse affect derived from an unevenness of air, i.e., variation of refractive index of air interposed between the beams. This in turn requires that the lens is inspected only within a chamber closely temperature controlled room. The interferometer using the shearing interference optical system requires that the optical component is moved so slowly to obtain interference figures. This means that a possible external vibration, for example, may move the optical component such as mirror to prevent the precise interference image from being obtained, deteriorating the reliability of the evaluation results. To this end, the interferometer should be placed on a vibration-free table, for example, in order not to receive the possible external vibrations.